Extremely useful card, easily better than Dr. Boom if you throw him behind a beefy taunt. The only reason Dr. is more widely used is because he makes an instant board impact, but if you're running a control deck, Troggzor the Earthinator is your go-to, guaranteed 2-for-1, if not a game winning, card. In my humble opinion at least.

Your 2nd point is seriously flawed. First of all 9/11 for 7 mana in 1 card is amazing (obviously there are better combinations of cards adding up to 7 mana cost that are better), and you forget that the 3/5 also discourages the use of spells or it'll grow. In the worst case your opponent is using 2 cards to kill it - one of which being a big spell if it can kill a 6 toughness creature - which is card advantage for you.

This card is fantastic when used optimally, the only spell it might be afraid of is Lightbomb because the rockjaw should be hit by it as well. Casting it naked is obviously a terrible play, but if you can get it out early via ramp or behind big taunts as pomonkeypo pointed out, you're in a nearly unbeatable position.

Remember when everyone thought this is the most OP card from the GvG set? And then StrifeCro said "Nah, I'm just gonna play Dr. Boom". And kids, that's how Dr. Boom became the most OP card in the game.